1. Field of the Invention
The present invention relates to a washer assembly for dental operations, which is designed to jet steam or boiling water so as to clean gypsum models of wax and stains deposited on their surfaces, wash metal frames prior to the placing of porcelanous materials, clean polishing tools of cuttings for clogging-preventing purposes, and so on.
2. Prior Art
In dental operations, washing should frequently be carried out. For instance, when dental prostheses are made on the basis of a prepared gypsum model, it is divided into denture molds by a saw, trimmed by a polishing tool or knife, marked by a color pencil or waxed up, so that it may be stained on its surface with gypsum powders, wax or dirt from the hands of an operator. Such a gypsum model should be washed out to clean its surface of stains. Prior to the placing of porcelanous materials, metal frames also have to be washed out to clean their surface of foreign matter for the purpose of bettering their adhesion to porcelanous materials. Similarly, polishing tools such as carbide bars or diamond points used to polish gypsum models, sintered porcelanous materials or waxes should be washed out to remove cuttings from them for clogging-preventing purposes.
For such washing, dental washers were used, which are of such a structure as set forth in Japanese Utility Model Laid-Open Nos. 197692/1988 and 201521/1988, wherein a washing liquid is heated in a heating vessel by an electrical heater to generate superheated steam and jet it through a gas/liquid separator. In such dental washers, the means for feeding the washing liquid into the heating vessel is provided by an electromagnetic pump of a structure in which first and second pump chambers are located between a suction inlet and a discharge outlet, valves are located on the suction sides of the pump chambers, the valves being normally engaged by springs to inhibit the suction of the washing liquid, and a plunger is passed into the first pump chamber to vary its volume by the magnetic force of an exciting coil. In the electromagnetic pump of such a structure, as the plunger is actuated by the magnetic force of the exciting coil to increase the volume of the first pump chamber, a negative pressure is generated in the first pump chamber to open the valve on its suction side and, hence, suck in the washing liquid. Then, as the plunger is actuated to decrease the volume of the first pump chamber, the internal pressure of the first pump chamber is so increased that the valve located on the suction side of the second pump chamber is opened to pump the washing liquid from the first pump chamber to the second pump chamber.
However, if the washing liquid fed into the electromagnetic pump contains air in the form of air bubbles, then the air is compressed or expanded in association with a change in the volume of the first pump chamber, even though the plunger is actuated. This in turn results in a vapor lock situation in which a reduction in the force for opening or closing the valve located on the suction side of the second pump chamber with the discharge pressure of the electromagnetic pump is considerably reduced to a minimum discharge pressure of about 0.3 kg/cm.sup.2 gauge.
Such dental washers are mainly used in prosthodontia clinics, and they make use of a heating vessel having a relatively small volume (on the order of about 400 cc at which sufficient outcomes are obtainable), since the amount of steam used for one washing may be reduced or limited. Thus, when the washing liquid is fed into the closed heating vessel, its internal pressure is so likely to rise because of its small volume so that, if the washing liquid fed into the electromagnetic pump contains air in the form of air bubbles, there is then a reduction in the difference between the pressure in the heating vessel and the discharge pressure of the electromagnetic pump, thus encountering vapor lock and difficulty in feeding the washing liquid into the heating vessel. Thus, not only is a considerable length of time needed to feed the desired amount of the washing liquid into the heating vessel, but it is also impossible to supply the washing liquid into the heating vessel, when the pressure in the heating vessel exceeds the discharge pressure of the electromagnetic pump.
In order to remove the defects of such a dental washer, there has been proposed such a steam washer unit as disclosed in Japanese Utility Model Laid-Open No. 197692/1988. This arrangement includes an air vent cock connected to a liquid supply pipeline extending between an electromagnetic pump and a check valve positioned on its discharge side. While air is expelled from within the electromagnetic pump, a washing liquid is pumped into a heating vessel by opening that air vent cock. This cock is manually opened or closed.
A problem with the dental washer of such a structure is that it needs some labor and so is inconvenient to operate. This is because when the washing liquid to be supplied contains air in the form of air bubbles, the air vent cock should be closed manually, after, while that cock is kept opened manually, the air bubble-containing washing liquid is pumped to fill a first pump chamber of the electromagnetic pump with the air bubble-free washing liquid alone, thereby recovering its discharge pressure. Another disadvantage is that when the air vent cock remains open inadvertently, a large amount of the washing liquid overflows the first pump chamber. In addition, when the first pump chamber of the electromagnetic pump is filled with the washing liquid containing air in the form of air bubbles during the operation of the dental washer, when the air vent cock remains closed inadvertently, it is impossible to pump the washing liquid into the heating vessel due to the elevated internal pressure in the heating vessel. It is then necessary to open the air vent cock manually to fill the first pump chamber of the electromagnetic pump with the washing liquid alone and thereafter close it manually. Thus, this unit is very difficult to deal with.